A recent test in Japan using environmental DNA (eDNA) to survey eel populations in rivers achieved similar detection rates to electro-shock fishing – without killing eels – and another test, on jack mackerel, improved estimation accuracy.
Molecular analysis of DNA left in the environment has proven to be an effective tool for detecting the presence of species, and is being applied more frequently in place of traditional survey fishing. It usually uses the polymerase chain reaction (PCR) process – a process also used in the COVID-19 antibody test – to rapidly duplicate genetic material to a quantity that can be effectively detected and identified. It has the advantage that it does not disturb or kill the fish – important for endangered species.
A group of scientists in Japan reported last year the accuracy of eDNA sampling for the presence, or absence, of Japanese eel in 10 river basins in Japan. They verified the accuracy by comparing results with those of electrofishing in the same areas. They also examined the relationship between the abundance of eDNA and the biomass of the eels.
The eDNA of Japanese eels was detected at 56 (91.8 percent) of the 61 study sites from which individuals were collected by electrofishing, and at an additional 35 sites where individuals were not collected, indicating that eDNA analysis has a greater sensitivity for detecting the presence of eels than electrofishing.
There was a weak association between the amount of eDNA and the biomass of eels. The authors attributed the weakness of the association to a few factors: the eDNA and electrofishing were carried out in different years and may reflect changes in populations from year to year; particularly large tissue fragments may have entered the water; or DNA trapped in sediments may have become resuspended in the water. Taking multiple samples at one site is a possible countermeasure.
Getting an accurate correlation between eDNA quantity and biomass has been a drawback in using the eDNA method. This issue was addressed by another Japanese team in a paper published 30 June, 2020. In this study, the team was able to obtain an estimate of the abundance of Japanese jack mackerel in Maizuru Bay using eDNA that was comparable to an estimate by the echo sounder method.
They did so by calculations accounting for many factors that affect the amount of eDNA, such as the fish size and shedding rate for the species as revealed in earlier tank studies. They used a method called the “Princeton Ocean Model (POM)” and applied it to the conditions of the locality to account for such factors as the depth of water, inflow of fresh water, and meteorological conditions.
Photo courtesy of USGS
